Choosing What to Back Up: A Guide to Home Battery Strategy

Installing a home battery is about more than just storing energy; it’s about resiliency. When the grid goes down, your battery becomes your personal power plant. However, in most cases, a battery won’t back up your entire home indefinitely (unless you want a system that’s too large and expensive for most of your needs). 

Designing the right battery system for your home is a matter of balancing what you want backed up and what you want to pay for. You won’t have to decide without help. Here’s a starting point.

Why do batteries typically back up only emergency loads?

“Load” is the industry term for any use of electricity. In a home it might be the EV charger, the microwave, or the security camera.

Most homeowners opt to back up emergency loads only rather than whole home backup. There are three main reasons for this.

• Capacity constraints: Batteries have a finite amount of stored energy (measured in kWh). High-draw appliances like central air conditioning, electric clothes dryers, or EV chargers can drain a standard battery quickly.
• Discharge rates: Some appliances require a large surge of power to start up. Your battery needs to be able to provide that instantaneous peak power.
• Cost efficiency: Sizing a system to run your entire home exactly as it functions normally can greatly increase the battery installation cost. By focusing on essential or emergency loads, you can get the most cost effective solution.

How long can a solar battery provide backup power?

The duration of your solar battery backup depends heavily on two factors: your solar generation and your power consumption. Here is how different load choices impact backup duration according to one solar design tool.

If you have a battery on the smaller side (5 kWh) and choose to back up your heat pump alongside your essentials, your estimated backup window is smaller because heating and cooling are energy-intensive.

• Loads: Refrigerator, WiFi, phone chargers, television, microwave, lighting, two laptops/PCs, and a heat pump.
• Sunny weather: 9+ hours
• Cloudy weather: 7 hours

By removing the heat pump from the backup circuit, your estimated runtime extends dramatically.

• Loads: Refrigerator, WiFi, phone chargers, television, microwave, lighting, two laptops/PCs, and NO heat pump.
• Sunny weather: 7+ Days (Solar recharges the battery faster than you use it)
• Cloudy weather: 18 hours

A larger battery (10+ kWh) paired with the same solar system and backing up the same devices (no heat pump) would last much longer.

• Sunny weather: An estimated 7+ days 
• Cloudy weather: 49 hours

Add the heat pump back into the mix, and the bigger battery still provides a significant amount of estimated backup time.

• Sunny weather: 7+ days
• Cloudy weather: 19 hours

According to Tesla’s public design tool, a Tesla Powerwall 3, a much larger battery with 13.5 kWh of capacity, paired with 5.7 kW of solar can provide the following average backup. (Tesla’s tool doesn’t specify what devices are backed up.

• Sunny weather: 7+ days of backup power.
• Average winter weather: Up to 18 hours.

Should you get a backup battery?

Deciding to invest in a battery depends on your specific goals:

• Grid reliability: If you live in an area prone to wildfires, hurricanes, or frequent blackouts a battery can help provide additional resiliency to your home.
• Financial incentives: In many regions, batteries allow you to avoid time-of-use (TOU) rates by powering your home with stored energy when utility prices are highest.
• Environmental impact: A battery ensures you are using 100% of the clean energy your solar panels produce, rather than sending it back to the grid.

The best way to understand how a battery will work with your specific home is to get an estimate from an expert. Advisors at Palmetto can help you decide how much battery capacity you need to meet your goals and whether a battery is the right fit at all. Reach out for an estimate today.

Frequently asked questions

Can I add more backup loads later?

It is usually easier to decide on your critical loads during the initial installation, as moving circuits later requires additional electrical work.

Will my solar panels work during a blackout if I don't have a battery?

No. For safety reasons, standard solar systems shut down during a power outage to prevent back-feeding electricity into the grid while utility workers are making repairs. You need a battery (and a transfer switch) to temporarily separate your home from the grid.

How long do these batteries last?

Most modern lithium-ion home batteries come with a 10-year warranty and are expected to maintain about 70% of their capacity at the end of that decade.